1. Field of the Invention
This invention relates to radiation sensors in general and more particularly to a radiation detection panel comprising a plurality of radiation sensors which include a dielectric layer with adjusted resistivity.
2. Description of Related Art
Radiation sensors able to convert incident radiation directly to an electrical charge indicative of the intensity of the incident radiation are known. Typically such sensors comprise a complex structure which includes a bottom and a middle conductive electrode separated by a dielectric to form a charge storing capacitor. A radiation detection layer, which may be a photoconductive layer, is placed over one of the electrodes. Over the photoconductive layer is another dielectric layer and a top electrode over the dielectric layer completes the sensors structure. Charge blocking layers are often provided between the conductive electrodes and the photoconductive layer.
A charging voltage is applied between the top electrode and the bottom capacitor plate.
Upon exposure to radiation, a charge proportional to the exposure level accumulates in the storage capacitor formed by the bottom and middle electrodes. Read-out of the stored charge is usually done by addressing the middle electrode and flowing the capacitor charge to a charge measuring device such as a charge integrating amplifier.
A plurality of such sensors may be assembled in an array of rows and columns to form a radiation detection panel. By sequentially reading out the charges accumulated in the individual sensors an image of the relative exposure of different areas of the panel is obtained. This image represents the radiation intensity incident on the panel after it has passed through a subject illuminated by the radiation. When the radiation is X-ray radiation and the subject is a patient the resulting image is a radiogram, captured as a plurality of charges. This radiogram can be displayed on a Cathode ray tube or other device for viewing.
The charge stored in the capacitor is read-out using a switch which connects, upon command, the middle electrode to the input of the charge measuring device. In practice such switch is usually an FET transistor created integrally with the sensor, having its source electrode connected directly to the middle electrode of the sensor. Both the drain electrode and the gate are accessible from outside the sensor. The drain is connected to the charge integrator. An electrical signal applied to the gate switches the transistor to a conductive state and permits the charge to flow from the capacitor to the integrator for detection.
The above described technology is well known in the art and well described in a number of publications and issued patents, exemplary of which is U.S. Pat. No. 5,319,206 issued Jun. 7, 1994 to Lee et al., and in an article by Denny L. Lee, Lawrence K. Cheung and Lothar S. Jeromin, entitled "The Physics of a new direct digital X-ray detector" appearing in the Proceedings CAR '95, Springer-Verlag, Berlin, pp. 83. Both the patent and the article are incorporated herein by reference.
The simplified sensor and transistor structure described above, while quite adequate, is, however, vulnerable to overexposure. The term "exposure" is used in this specification to designate the product of the intensity of the incident radiation times the time during which the radiation impinges on the sensor.
When the detector is exposed to radiation, electron-hole pairs are generated in the radiation detection layer which, under the influence of the electric field produced by the applied charging voltage, travel toward the top and middle electrodes respectively. If such charges are allowed to flow freely, the charge stored in the capacitor formed between the middle and bottom electrodes increases continuously. The result of such continuous charge increase is an associated voltage increase on the voltage appearing on the middle electrode which will eventually result in the catastrophic failure of the associated transistor switching element connected to the charge storage capacitor. The presence of a dielectric layer between the radiation detection layer and the top electrode eliminates this risk by presenting a barrier to the migrating charges, which begin accumulating in the interface between the radiation detection layer and the dielectric layer. These accumulated charges set up a secondary field opposing the applied charging field thus inhibiting further charge migration and providing a limit to the rising voltage on the middle electrode.
While this is an acceptable solution to the overexposure problem, the accumulated charges on the dielectric/photoconductor interface will interfere with subsequent exposures of the sensor. In order to eliminate the effect of such residual charges, there is usually required an additional step in which the trapped charges are eliminated. This extra step is not only time consuming, but, for the reasons discussed later, inhibits the use of this type of sensor for continuous, real time imaging, such as in fluoroscopy applications.
It is an object, therefore, of the present invention to provide a sensor, and an associated panel comprising a plurality of such sensors, which is protected from catastrophic failure due to overexposure during single exposure operation, and which still has fast response for use in real time viewing applications.